1. Field of the Invention
The invention relates in general to transmission of a certain sequence of symbols. In particular the invention relates to diversity transmissions where the symbols belonging to the sequence are sent using at least two antennas.
2. Brief Description of Related Fields
In cellular networks the downlink and uplink radio transmissions comprise synchronization channels, which can be special synchronization symbols. Using the information carried in the synchronization symbols, for example, the receiver can determined the timing of the transmission. Information is usually sent in frames, and the frames consist of a certain number of time slots. The time slots, in turn, consist of a certain number of symbols. If synchronization symbols are used, they can be sent, for example, once in each time slot. It is also possible to send synchronization information in bursts, so that more information is sent at a time, but synchronization information is sent less frequently than once in a time slot. From the synchronization information it is possible to determine both the time slot timing and the frame timing, i.e. where time slots and frames start.
The synchronization symbols may carry also other information than just indicate timing. For example, in Wideband Code Division Multiple Access (WCDMA) cellular networks the synchronization symbols carry certain information about the spreading code that a base station uses to spread the downlink transmissions. In a handover, for example, the mobile station entering a new cell can determine the part of the downlink spreading code with the help of the synchronization symbols. The mobile station needs to know the spreading code in order to find out the control information transmitted via the common control channel. Otherwise it cannot, for example, communicate with the radio access network after power up or in a handover situation receive from the new cell cell-specific control information that is needed to perform the handover.
Traditionally information is transmitted over radiolink using a single antenna. Transmission diversity refers to sending information via more than one antenna. The transmitted information can be, for example, encoded so that the transmitted symbol flows are not equal, but the original information flow can be determined from each transmitted symbol flow. The receiver can, for example, choose special decoding scheme in case transmission diversity is used and deduce the transmitted information. The synchronization symbols can carry information also about the use of some transmission diversity scheme. It is important that the receiver can determine the sent synchronization symbol correctly. Otherwise, for example, it may fail to identify the transmission diversity and encoding schemes that are used.
FIG. 1 represents a typical WCDMA cell 100, where there is a base station 101 in the middle of the cell. There are also two mobile stations 102 and 103 in the FIG. 1, and the communication between each mobile station and the base station is indicated with arrows. The base station broadcasts common control information to all the mobile stations in the cell, and it spreads this common control information with a certain spreading code. In a WCDMA system, a spreading code usually consists of two parts: a long scrambling code CS and a short channelization code CC. The scrambling code is effective to eliminate, for example, the effect of multipath propagation. The channelization codes that are used within a cell are orthogonal, and they are effective to distinguish, for example the transmission to each mobile station. In a WCDMA system, within a cell a same scrambling code CS may be used for all downlink transmissions. The downlink transmission are synchronized, and therefore the different channelization codes are enough for successful despreading of the transmitted signals. In the neighboring cells, other scrambling codes are used so that adjacent cells do not disturb each other's transmissions.
The use of spreading codes in downlink transmission is presented in FIG. 1, where the arrow 111 represents the common control information broadcast. The spreading code can be presented as the product of the scrambling and channelization codes CCC=CS CC. When entering a new cell, the mobile station can determine the downlink scrambling code CS from the broadcast transmission the base station sends. The channelization code related to common control information is typically a fixed constant throughout the WCDMA system, so after determining the downlink scrambling code and the frame timing, the mobile station can determine the common control information.
The arrow 112 in FIG. 1 represents the downlink transmission to the mobile station 102, and the arrow 113 represents the downlink transmission to the mobile station 103. The spreading code CD1 for the downlink connection to the mobile station 102 is CD1=CS CC1, and the spreading code CD2 for the downlink connection to the mobile station 103 is CD2=CS CC2. Since the uplink transmissions are not synchronized and each mobile has its own radio channel from the mobile station to the base station, each mobile station may use a specific scrambling code, and various channels, for example, to a certain mobile station may be separated using various channelization codes. The downlink and uplink spreading codes for connections terminating to a mobile station are usually established either when a mobile station enters a new cell or when a new connection is established between the mobile station and the radio access network.
FIG. 2 shows some of the common channels a base station in a WCDMA system generally transmits The pilot symbols are transmitted over a common pilot channel (CPICH) 201. The pilot symbols are usually sent 100% of the duty cycle. The pilot symbols are predetermined, and CPICH is spread using the downlink scrambling code CS and a fixed channelization code.
The synchronization channel (SCH) 202 occupies typically 10% of the duty cycle in the beginning of each time slot 210. The frame 211, which comprises a certain fixed number of time slots, is also presented in FIG. 2. The synchronization channel carries two synchronization codes: a primary synchronization code 203 and a secondary synchronization code 204. These codes are transmitted simultaneously within one symbol period. Both the primary and secondary synchronization codes can be modulated, for example, with the same symbol, and because the codes have good crosscorrelation properties the receiver can distinguish the codes. A mobile station entering a new cell or measuring a new cell in the neighborhood may always receive successfully information broadcast over the SCH.
The primary synchronization code is a constant code that denotes the beginning of the time slots. The secondary synchronization codes, which form a synchronization code sequence or word, indicate the timing of the frames. In addition to the frame timing, the second synchronization code sequence within a frame indicates the scrambling code group to which the downlink scrambling code the base station uses belongs. A mobile station entering a new cell may determine the downlink scrambling code, for example, by testing the scrambling codes of the indicated scrambling code group on the CPICH. The correct scrambling code CS is the one that with the known channelization code produces from the received radio signal the known transmitted pilot symbols.
Once the scrambling code CS has been determined, the received pilot symbols may be used, for example, for determining the complex channel coefficient. In general, the radio signal that is received is not exactly the same as the transmitted one. The signal may experience changes in amplitude and phase, and these changes are time-dependent. They are taken into account using the complex channel coefficient h when the despread signal is processed. An estimate ĥ for the channel coefficient can be determined by comparing the received pilot symbols to the known transmitted pilot symbols. The channel coefficient may be assumed to be constant during the time over which the pilot symbol and the studied symbol are transmitted.
Common control information is transmitted using, for example, a Primary Common Control Physical Channel (PCCPCH) 205. PCCPCH is transmitted 90% of the duty cycle, at the time when the synchronization symbols are not sent. It is spread using a predetermined channelization code and the downlink scrambling code, as discussed above. After the scrambling code has been identified, the mobile station may despread the CCPCH information from the spread signal it receives. The information may be, for example, information related to the logical Broadcast Control Channel (BCCH). The mobile station needs the BCCH information, for example, to start communicating with the radio access network after power up or to make a successful handover.
FIG. 2 represents a situation where the base station uses only one antenna TX1 for broadcasting information. When transmission diversity is employed, there are at least two antennas where the information may be transmitted. It is preferable that each antenna transmits its own pilot signal, so that the channel coefficient estimates can be determined for each antenna. The radio waves emitted for the two transceivers may propagate in different ways to the antenna of the mobile station.
FIG. 3 represents some broadcast channels when transmission diversity and two antennas TX1 and TX2 are in use. The antenna TX1 transmits the common pilot channel CPICH 201 similarly as when no transmission diversity is employed. The antenna TX2 transmits an auxiliary pilot 301. The synchronization symbols may be transmitted using only one antenna or both antennas. In time switched transmit diversity (TSTD) both antennas are used to transmit the symbols, one at a time. FIG. 3 shows how the synchronization symbols are transmitted using TSTD and an alternating transmission pattern. For example, the synchronization symbol 302 is transmitted from the antenna TX1 and the synchronization symbol 303 is transmitted from the antenna TX2. Each synchronization symbol carries both the primary and the secondary synchronization code.
The common control information may be also transmitted from both antennas TX1 and TX2. In this case the BCCH information, for example, is encoded before it is transmitted over the PCCPCH channel. Space time transmit diversity (STTD), for example, specifies that from the primary antenna TX1 the symbols are transmitted as such, i.e. the sequence of transmitted symbols is S1, S2, S3, S4, . . . . From the second antenna TX2 the sequence of transmitted symbols starts in the following way: −S2*, S1*, −S4*, S3*, . . . , where the asterisk indicates the complex conjugate. FIG. 3 presents the PCCPCH data 304 transmitted from the antenna TX1 and the PCCPCH data 305 transmitted from the antenna TX2. It is possible also to use the space time transmit diversity for the BCCH information but transmit all the synchronization symbols from one antenna.
The base station may indicate the use of a diversity scheme and two transceivers, for example, by transmitting a specific message on a broadcast channel or modulating the synchronization symbols. A certain synchronization symbol value indicates that the STTD is on, and another value indicates that it is off. The mobile station may also determine the use of a diversity scheme by detecting the auxiliary pilot symbols. The mobile station may also use all three indicators of the diversity scheme.
When the mobile station detects the presence of STTD using the synchronization symbol, the value of the synchronization symbol needs to be determined reliably. When a certain symbol needs to be determined, the effect of the channel coefficient has to be taken into account. The mobile station receives the following signal rr=hsSCH+n where h represents the complex channel coefficient, sSCH represents the synchronization symbol and n represent the noise.
When the received signal r in multiplied by the complex conjugate of the channel coefficient estimate ĥ*ĥ*r=ĥ*(hsSCH+n)=ĥ*hsSCH+ĥ*n the result is the synchronization symbol scaled with a scalar ĥ*h and the term related to noise. From here it is quite straightforward to infer the value of the synchronization symbol.
Above, the synchronization symbols have been used as an example of a sequence of symbols that is transmitted using two antennas. The problem is that when the TSTD diversity scheme is in use, the mobile station cannot necessarily distinguish from which antenna a certain synchronization symbol, or any other symbol that is transmitted using a time switched diversity scheme, is transmitted. Consider, for example, a situation where a certain sequence of symbols is transmitted once is every time slot, and a frame consists of an odd number of time slots. If the symbols belonging to the sequence are transmitted using a time switched diversity scheme, two diversity antennas are used and the transmission pattern is an alternating pattern, in a certain time slot the symbol belonging to the sequence is transmitted from one antenna in every other frame and in the rest of the frames from the other antenna. Therefore the mobile station does not know, which channel coefficient estimate to use for a symbol sent in a certain time slot with a time switched transmission scheme.
To obtain a reliable result, the signal transmitted by the primary transceiver has to be processed with the channel coefficient estimate ĥ2 determined from the primary pilot and the signal transmitted by the secondary transceiver has to be processed with the channel coefficient estimate ĥ2 determined from the auxiliary pilot. Not knowing from which antenna a certain symbol is transmitted causes unnecessary interference to the decision determining which symbol was sent. In case of synchronization symbols, this may cause that the mobile station cannot utilize the transmission diversity of, for example, the common control information for enhancing the quality of the received signal. Consequently, if the transmission diversity is in use, but the receiver does not notice this, the quality of the received common control signal may be poorer than in a case where no transmission diversity is applied.
The object of the invention is to provide a versatile method for transmitting a sequence of symbols using at least two antennas. A further object of the invention is that the method enables to determine unambiguously from which antenna a symbol belonging to sequence is transmitted.
The objects of the invention are achieved by starting the time switched transmit diversity pattern of the sequence of symbols always from the same antenna in the beginning of a frame and by using the same pattern in each frame.